Blood coagulation is a complex chemical and physical reaction that occurs when blood (herein, “blood” shall mean whole blood, citrated blood, platelet concentrate, plasma, or control mixtures of plasma and blood cells, unless otherwise specifically called out) comes into contact with an activating agent, such as an activating surface or an activating agent. In accordance with one simplified conceptual view, the whole blood coagulation process can be generally viewed as three activities: platelet adhesion, platelet aggregation, and formation of a fibrin clot. In vivo, platelets flow through the blood vessels in an inactivated state because the blood vessel lining, the endothelium, prevents activation of platelets. When a blood vessel is damaged, however, the endothelium loses its integrity and platelets are activated by contact with tissue underlying the damaged site. Activation of the platelets causes them to become “sticky” and adhere together. Additional platelets then adhere to the activated platelets and also become activated. This process continues until a platelet “plug” is formed. This platelet plug then serves as a matrix upon which blood clotting or coagulation proceeds. Blood “coagulation” and “clotting” are used interchangeably herein unless specifically distinguished from one another.
If the chemical balance of the blood is suitable, thrombin is then produced that causes fibrinogen to convert to fibrin, which forms the major portion of the clot mass. During clotting, additional platelets are activated and trapped in the forming clot, contributing to clot formation. As clotting proceeds, polymerization and cross-linking of fibrin results in the permanent clot. Thus, platelet activation plays a very important function in blood coagulation.
The clinical assessment of clotting function has long been recognized to be important in the management of surgical patients. Preoperatively, the assessment of the clotting function of the patient's blood is utilized as a predictor of risk of patient bleeding, allowing advanced preparation of blood components. Perioperative monitoring of the clotting function of the patient's blood is also important because coagulopathies can be induced by hemodilution of procoagulants, fibrinogen and platelets, by consumption of coagulation factors during surgical procedures, or by cardiopulmonary bypass. Post-operative assessment of clotting function is also crucial to the patient's successful recovery. For example, 3-5% of cardiopulmonary bypass patients require surgical reoperation to stop bleeding. Prompt assessment of clotting function could rule out coagulopathy as the cause of bleeding and could avoid unnecessary surgery that adds to patient morbidity and treatment costs.
Several tests of coagulation are routinely utilized to assess the complicated cascade of events leading to blood clot formation and test for the presence of abnormalities or inhibitors of this process. Among these tests are platelet count (PLT), thrombin time (TT), prothrombin time (PT), partial thromboplastin time (aPTT), activated clotting time (ACT), fibrinogen level (FIB), fibrinogen degradation product concentrations, and general purpose clotting (GPC) time. The aPTT test can be used to assess the degree of anticoagulation resulting from heparin administration. The PT test results can indicate the level of anticoagulation produced by warfarin (coumadin) administration. In the GPC test, the blood sample is exposed to calcium as an activating agent, but other reagents can be added to conduct the aPTT test, for example.
The ACT tests are further differentiated into high range ACT (HRACT), low rate ACT (LRACT), recalcified ACT (RACT), and HRHTC tests. The HRACT test features a slope response to moderate to high heparin levels in whole blood drawn from a patient during cardiac surgery for use in whole blood coagulation time tests. The LRACT test features a more sensitive slope response to heparin levels ideal for use in whole blood coagulation time tests run in the ICU, CCU and during dialysis. The RACT test is similar to the LRACT test, but the test is conducted on a citrated blood test sample, rather than a whole blood sample, and is typically conducted at a central testing station remote from the patient the test sample is drawn from.
The HRHTC test is a variation upon the HRACT test where an enzyme, heparinase, is added as a deactivating or neutralizing reagent to one test sample of the whole blood drawn from the patient to neutralize any heparin therein and thereby provide a heparin-free blood test sample. In this test, comparative coagulation times are determined from the heparinase-exposed blood test sample and the unexposed blood test sample. The two ACT test times will be close if no heparin is present in the whole blood drawn from the patient. The ACT test conducted on the heparinase-exposed blood test sample will result in a shorter coagulation test time than the ACT test conducted on the unexposed blood test sample, thereby establishing that the patient's blood contains at least some level of heparin.
During heart bypass surgery, the platelets of blood circulated in an extracorporeal circuit may become activated by contact with the materials present in the extracorporeal circuit. This activation may be reversible or irreversible. Once platelets are irreversibly activated, they lose their ability to function further. A deficiency of functional platelets in the blood may be indicative of an increased probability of a post-operative bleeding problem. Such a deficiency, and the resulting post-operative bleeding risk, could be remedied by a transfusion of platelet concentrate. Platelet functionality tests, e.g., the ACT test, can identify a deficiency of platelets or functional platelets and aid the attending surgeon in ascertaining when to administer a platelet concentrate transfusion. Such a test is further useful in ascertaining the efficacy of a platelet transfusion. By performing the platelet functionality test following a platelet transfusion, it is possible to determine if additional platelet concentrate transfusions are indicated. Real-time assessment of clotting function at the operative site is preferred to evaluate the result of therapeutic interventions and also to test and optimize, a priori, the treatment choice and dosage.
A number of different medical apparatus and testing methods have been developed for measuring and determining platelet activation and coagulation-related conditions of blood that can be used in real time during surgery, particularly bypass surgery, on fresh drawn blood samples or that can be used after some delay on citrated blood samples. Some of the more successful techniques of evaluating blood clotting and coagulation of fresh or citrated blood samples employ plunger techniques disclosed in commonly assigned U.S. Pat. Nos. 4,599,219, 4,752,449, 5,174,961, 5,314,826, 5,925,319, and 6,232,127, for example. These techniques are embodied in the ACT II® automatic coagulation timer, commercially sold by the assignee of this patent application.
In U.S. Pat. No. 5,302,348, an apparatus and method are disclosed for performing a coagulation time test on a sample of blood deposited in a fluid reservoir of a disposable cuvette. A capillary conduit having at least one restricted region is formed within the cuvette. The cuvette is inserted into a testing machine that engages the cuvette and draws blood from the fluid reservoir into the capillary conduit. The blood is then caused to reciprocally move back and forth within the capillary conduit so that the blood is forced to traverse the restricted region. Optical sensors of the testing machine are employed to detect movement of the blood. The testing machine measures the time required each time the blood is caused to traverse the restricted region. Coagulation is considered to have occurred and the overall coagulation time is displayed to the operator when a measured time is a predetermined percentage longer than an immediately preceding time.
In U.S. Pat. No. 5,504,011, a similar apparatus and method are disclosed for performing multiple coagulation time tests on a sample of blood deposited in a fluid reservoir of a disposable cuvette having multiple capillary conduits within the cuvette. Each of the conduits contains a dried or lyophilized activation reagent that is rehydrated by the blood. The blood in each conduit is then reciprocally moved across a restricted region of the conduit until a predetermined degree of coagulation occurs. Since the coagulation time is being monitored in multiple conduits, a representation coagulation time for a given sample can be determined. A normalizing control agent is present in at least one of the conduits. The normalizing control agent counteracts any effects of anticoagulants present in the blood sample, thereby allowing the blood sample to have generally normal coagulation characteristics. The normalized blood is tested simultaneously with the untreated blood to provide a reference value against which the functionality of the test system and the quality of the sample can be judged.
The apparatus and methods disclosed in the '348 and '011 patents only check the state of the sample during the reciprocal back and forth movement of the sample through the restricted region capillary. The detection of coagulation would be delayed or inaccurate if the sample coagulates between movement cycles.
In U.S. Pat. No. 6,200,532, a device and method for performing blood coagulation assays, particularly prothrombin times and activated partial thromboplastin times and other clotting parameters are disclosed. One embodiment of the device comprises a disposable cassette containing a sample inlet for sample delivery, a pair of interleaved spiral capillary channels for driving force, and a reaction chamber with an appropriate dry reagent for a specific assay, and a piezoelectric sensor. The device could also include a heating element for temperature control, and a magnetic bender. Compressed air is employed to drive the sample into the two spiral capillary channels. The magnetic bender is driven by an electromagnetic field generator and is attached onto a piezoelectric film in contact with the blood sample. The electric signal generated in the piezoelectric film is characterized by its frequency and amplitude due to the movement of the attached metal film. The signal collected at the site of the piezoelectric film represents the process of a biochemical reaction in the reaction chamber as the blood sample proceeds to the point at which clot formation starts and is amplified by an amplifier and rectified into a DC voltage and is sent to a recording unit and/or display unit.
A method and apparatus for measuring coagulation of blood is disclosed in U.S. Pat. No. 4,879,432, wherein a timer is started as a stream of solid particles are introduced into a tube containing the blood sample that descend through the blood sample under the force of gravity. The particles are micron-sized grains of glass, for example, that are highly wettable and can descend through the blood sample until stopped by the fibrin network that forms as coagulation begins. Photoelectric cells detect the stopping of the particles, and the coagulation time is determined.
Other approaches to the detection of a change in viscosity of a fluid, particularly changes accompanying fibrin formation and clotting in blood samples, have been disclosed involving ferromagnetic elements or materials introduced in the fluid that are inhibited from moving freely when the viscosity change or clotting occurs.
For example, a method and apparatus is disclosed in U.S. Pat. No. Re. 27, 866 wherein the coagulation of a blood plasma sample is intensified by dispersing iron oxide particles throughout the plasma sample. A rotating magnetic field is applied that causes the particles to move within the sample and activate the clotting reaction. The moving particles collect fibrin strands that are formed, thereby changing the optical properties of the particle-sample mixture, and the changed optical properties are detected optically.
Further methods and apparatus employing magnetic effects for determining changes in viscosity of a fluid, particularly a blood sample, within a test tube are disclosed in U.S. Pat. Nos. 3,635,678, 3,836,333, 3,967,934, and 5,145,082. In the '678 and '934 patents, a steel ball within the fluid is maintained at a certain location in the test tube by a magnetic field, while the tube is moved up and down. The magnetic force is insufficient to attract the ball when the fluid in the tube changes viscosity, and the failure of the steel ball to stay within the magnetic field is detected photoelectrically. In the '333 and '082 patents, a blood sample is placed in a test tube along with a ferromagnetic member. A magnetic field is applied to the ferromagnetic member, and the applied magnetic field maintains the ferromagnetic member oriented with the magnetic field as the test tube is rotated about its axis. The magnetic force is insufficient to attract the ball when the fluid in the tube changes viscosity, e.g., due to clotting, and the failure of the member to stay within the magnetic field is detected magnetically with a reed switch.
In other approaches to measuring coagulation time, a blood sample is disposed in a test chamber having a ferromagnetic element therein. The ferromagnetic element is raised against gravity, and the field is discontinued so that the ferromagnetic member drops through the sample in the test chamber.
For example, a viscometer is disclosed in U.S. Pat. No. 4,648,262 wherein the fluid sample under test is placed within a capillary tube along with a metal ball. A magnet mounted on a rotating drum periodically raises the ball to the top of the tube and then releases the ball so that it can fall to the bottom of the tube. The viscosity of the fluid is determined by the rate of descent of the ball in the tube.
Apparatus and methods for detecting changes in human blood viscosity are disclosed in U.S. Pat. Nos. 5,629,209 and 6,613,286, wherein heparinized blood is introduced into a test cartridge through an injection port and fills a blood receiving/dispensing reservoir. The blood then moves from the reservoir through at least one conduit into at least one blood-receiving chamber where it is subjected to a viscosity test. A freely movable ferromagnetic washer is also located within the blood-receiving chamber that is moved up using an electromagnet of the test apparatus and allowed to drop with the force of gravity. Changes in the viscosity of the blood that the ferromagnetic washer falls through are detected by determining the position of the ferromagnetic washer in the blood-receiving chamber over a given time period or a given number of rises and falls of the ferromagnetic washer. The blood sample can be mixed with a viscosity-altering agent (e.g., protamine) as it passes through the conduit to the blood-receiving chamber. Air in the conduit and blood-receiving chamber is vented to atmosphere through a further vent conduit and an air vent/fluid plug as the blood sample is fills the blood-receiving chamber.
The movement of the washer in the above approach is actively controlled only when it is moved up, and the washer passively drops with the force of gravity. The washer is free to float in the test chamber and may drift side-to-side as it is moved up or floats downward. The side-to-side drifting movement may affect the rise time and the fall time, which could add error to the coagulation time measured. The washer may eventually stop moving as a clot forms about it, and no additional information can be obtained on the coagulation process in the sample.
It would be desirable to provide inexpensive, relatively simple, easy to use, and accurate equipment and techniques that quickly measure one or more of the aforementioned blood coagulation times, including TT, PT, aPTT, and ACT.